1. Field of the Invention
The present invention relates to a brake assembly for a vehicle such as a snowmobile and, more particularly, to a liquid cooled brake caliper.
2. Discussion of the Related Art
Caliper disc brakes generally include a caliper housing having a brake pad assembly supported in the housing on each side of a disc brake rotor. Typically, both brake pad assemblies are mounted on movable pistons that can be mechanically or hydraulically driven into engagement with the rotor. Alternatively, one of the brake pad assemblies could be driven into engagement with the rotor, and the other brake pad could be pulled into engagement with the rotor due to the caliper housing movement or due to deflection of the brake disc.
Brake calipers must be capable of withstanding the heat created by the friction of brake pads rubbing against the brake disk. When used in high speed, high torque, and/or high duty cycle applications such as high performance snowmobiles, brake calipers tend to overheat because of the large quantities of energy that must be absorbed by the brakes during braking, often causing the brake fluid to boil. Regular brake fluid boils at about 350xc2x0 F. and high-temperature brake fluid boils at from 400 to 500xc2x0 F. However, brake fluid absorbs water, which lowers its boiling point. Over time, the boiling point of brake fluid containing water may drop to virtually the boiling point of water, e.g., to around 230xc2x0 F. to 250xc2x0 F. In situations where brake fluid boils, brake life is adversely affected, often resulting in damage to the brakes.
Attempts have been made to address the overheating of brake fluid. In one early design developed by the assignee of the present application, an extruded aluminum manifold was inserted between the pad and the piston of a brake caliper. Water was pumped through the manifold to directly cool the pad and piston. This system was manufactured only with considerable difficulty and expense because extruded aluminum had to be extrusion-bent, had welded end caps, yet still had to be watertight. It was also relatively heavy
In another, later system manufactured by the assignee of the present application, the assignee took advantage of the good thermal conductivity of an aluminum brake caliper housing to cool the piston indirectly via conductive heat transfer with a liquid coolant, thereby negating the need to produce a complex manifold to supply coolant directly to the piston. In this system, an aluminum water cooled housing was mounted on the top of the main caliper housing. Several longitudinal cavities were formed in the top of the main caliper housing in fluid communication with first and second lateral cavities in the water cooled housing. Coolant inlets and outlets in the caliper housing opened axially into the first and second axially-offset lateral passages in the water cooled housing. The lateral passages in the water cooled housing were separated by baffles to promote water circulation through the longitudinal cavities in the main caliper housing. With this arrangement, water entering the inlet port of the water cooled housing flowed into the first lateral passage and was deflected through all three longitudinal cavities in the main caliper housing by baffles that separated the cavities from one another. The water then flowed into the second lateral passage in the water cooled housing and was directed back to the engine coolant system via the outlet opening.
The arrangement described above effectively cools the caliper housing but has several disadvantages. The baffled main caliper housing, though easier to manufacture than the earlier system described above, is still relatively complex and expensive to manufacture. It is also relatively heavy, undesirably contributing to a reduced acceleration-to-weight ratio in the vehicle serviced by the brake caliper. In addition, the convoluted now path of the fluid flow through the water cooled housing results in turbulent flow and considerably restricts fluid flow through the housing. More engine horsepower therefore is required to run the water pump at an effective rate than if the liquid flow were laminar and unrestricted.
Therefore, it would be desirable to provide a liquid cooled brake caliper that is less expensive and easier to manufacture than earlier known liquid cooled brake calipers. It would also be desirable to reduce the cost of manufacturing a liquid cooled brake caliper. It would also be desirable to reduce the weight of a liquid cooled brake caliper.
The invention, which is defined by the claims set out at the end of this disclosure, is intended to solve at least some of the problems noted above. The present invention provides a brake caliper that includes a cast metal housing, at least one piston slidably mounted in the bore, and a brake pad mounted on the piston. The housing has a bore formed therein and a coolant passage formed therethrough. The coolant passage is a single-pass passage having a coolant inlet and a coolant outlet. The coolant passage is fluidically isolated from the bore. The resultant caliper not only provides surprisingly effective conductive heat transfer from the hot brake fluid in the caliper, but is also easy to manufacture from a single casting and imposes much less flow restriction on coolant flowing therethrough than in prior systems.
A braking system for a vehicle is also provided. It includes a master cylinder, a source of liquid coolant, a rotatable disc, and a liquid cooled caliper having at least some of the characteristics described above. The cast metal housing of the liquid cooled caliper has a bore formed therein in fluid communication with the master cylinder. The coolant inlet and a coolant outlet of the coolant passage are connected to the source of liquid coolant.
A method of braking a vehicle is also provided. Brake fluid is admitted into a bore of a caliper housing to drive a pad on a piston into frictional contact with a rotating disc. Heat is generated as a result of the frictional contact. The brake fluid is cooled by directing a liquid coolant through a coolant passage in the caliper housing without changing directions of liquid flow within the caliper housing.